LIGHT 25.1

Chapter Twenty-Five: Light

25.1 Properties of Light

25.2 Color and Vision

25.3 Optics

Chapter 25.1 Learning Goals

Describe the properties of light.

Explain the relationship between energy and the colors of light.

Describe waves included in the electromagnetic spectrum in terms of energy, frequency, and wavelength.

Investigation 25A

Key Question: What happens when you mix different

colors of light?

Color

25.1 Properties of lightLight travels fast over

long distances and carries energy and information.

Light travels in straight lines, but can be bent by lenses or reflected by mirrors heat and warmth.

Light has color and can be bright or dim.

25.1 The electromagnetic spectrum

Light, like sound and heat, is a form of energy.

The visible light we see is part of the electromagnetic spectrum.

25.1 Properties of lightYou see book pages because light in

the room reflects from the page to your eyes.

Your eyes and brain use the information carried by the light to make a mental picture.

25.1 Light is produced by atomsMost light is produced by atoms.

When you put some energy into the atom, it excites the atom’s electrons.

Light is produced when the electron releases this energy.

25.1 Incandescent lightMaking light with heat is

called incandescence.

Atoms in the filament convert electrical energy to heat and then to light.

Incandescent bulbs are inefficient, but their waste heat can be useful.

25.1 Fluorescent lightTo make light,

fluorescent bulbs use high-voltage electricity to energize atoms of gas in the bulb.

These atoms release the electrical energy directly as light (not heat), in a process called fluorescence.

25.1 Color and energyWhen all the colors of the rainbow are

combined, we see light without any color.

We call the combination of all colors white light.

25.1 Color and energyCompare the hot, blue flame from a gas

stove to the orange flame of a match.

The light from a gas flame is blue (high energy) and the light from a match is red-orange (low energy).

25.1 The speed of lightThe speed at which light travels through

air is about 300 million meters per second.

The speed of light is so important in physics that it is given its own symbol, a lower case “c”.

25.1 Speed of lightThe speed at which electromagnetic

waves travel through air is about 300 million meters per second.

The speed of light is so fast that when lightning strikes a few miles away, we hear the thunder after we see the lightning.

25.1 Wavelength and Frequency of LightBecause the wavelength of light is so small, scientists measure it in nanometers.

One nanometer (nm) is one billionth of a meter (0.000000001 m).

25.1 What kind of wave is light?A sound wave is a oscillation of air.

A water wave is an oscillation of the surface of water.

An oscillation of electricity or magnetism creates electromagnetic waves.

25.1 Electromagnetic wavesIf you could shake the magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

25.1 Electromagnetic spectrumThe entire range of electromagnetic

waves, including all possible frequencies, is called the electromagnetic spectrum.

This spectrum includes visible light and invisible waves: radio wave microwaves infrared light ultraviolet light X-rays gamma rays